Outside panels of automobiles, bicycles, electrical appliances, etc. which require an aesthetic appearance are generally finished with thermosetting paints diluted with organic solvents. In many cases, this is accomplished by a so-called 2-coat 1-bake process which comprises applying a cationic electrodeposition paint to a substrate for corrosion resistance, applying an intermediate paint to secure weatherability, curing the two coated films by heating, further applying an organic solvent-base thermosetting enamel paint containing a colored pigment and/or a metallic pigment (to be referred to as the "base coat") as a top coat, air-drying the top coat, further applying an organic solvent-base thermosetting clear paint to the top coat, and then curing the two coated films simultaneously by heating.
In recent years, there has been an increasing demand for more improved appearances on finishing, for example smoothness, distinctness of image gloss, fatness, resources saving or pollution control, and also for lower coating costs.
With regard to improvement of finished appearances, measures have been taken by adding various rheology controlling agents so as to smoothen top coats, or by polishing intermediate coats. There has also been an attempt at utilizing high-solids paints for pollution control.
The addition of rheology controlling agents in the aforesaid process, however, is not a complete solution to the aforesaid problem because the improvement of smoothness and distinctness of image gloss attained by this method is limited. On the other hand, polishing of the intermediate coats increases the number of process steps and cannot sufficiently improve the finished appearance. Furthermore, conventional organic solvent-base base coats contain large amounts of organic solvents, and this is undesirable from the standpoint of resources saving and pollution control. The reduction of the amount of organic solvents in high-solids paints is naturally limited, and when these paints are used as base coats, whiteness on finish is difficult to obtain in regard to pale-colored metallic paints such as silver-colored paints. On the other hand, a coating process is available which comprises applying a cationic electrodeposition paint, an intermediate paint, a water-base metallic base paint and a powder clear paint. But this coating process has the defect that the number of required steps is large, and the total cost runs high.
The present inventors have extensively made investigations in order to develop a coating method which eliminates the various defects of the prior art described above, has a reduced number of coating steps, gives coated films of a very good finished appearance and can advantageously lead to resources saving and pollution control.